Semiconducting ceramic compositions

ABSTRACT

Ceramic semiconducting compositions are provided herein in which compositions consist primarily of BaTiO3 and minor amounts of Bi2O3 and TiO2 in the proportions defined in FIG. 1 of the drawings under the Region described by line A-B-C-D-E-F-G and which compositions further contain 0.01 to 4 mol % of MnO2. The disclosure also provides for a method of producing compositions having the aforementioned proportions by mixing the oxides defined by FIG. 1 with MnO2, pressing the ceramic composition into the desired shape and then sintering the compositions in air, followed by reducing the compositions in a reducing atmosphere at elevated temperatures. Ceramics produced from the compositions described above exhibit a large capacitance among other desirably electrical properties and may be conveniently produced in a small compact form.

United States Patent Ueoka et al.

[ June 27, 1972 [54] SEMICONDUCTING CERAMIC COMPOSITIONS [72] Inventors:I-lisayoshi Ueoka, Ichikawa-shi; Kazuo l'lorii, l-lunabashi-shi;Kazumasa Umeya,

lchikawa-shi, all of Japan [73] Assignee: TDK Electronics Co. Ltd.,Tokyo, Japan |22| Filed: Oct. 9, 1969 [21 1 Appl. No.: 865,031

Primary Examiner-Douglas J. Drummond Attorney-Wenderoth, Lind & Ponack[57] ABSTRACT Ceramic semiconducting compositions are provided herein inwhich compositions consist primarily of BaTiO and minor amounts of Bi,,O and TiO in the proportions defined in FIG. 1 of the drawings underthe Region described by line A-B-C- D-E-F-G and which compositionsfurther contain 0.0] to 4 mol of MnO The disclosure also provides for amethod of producing compositions having the aforementioned proportionsby mixing the oxides defined by FIG. 1 with MnO pressing the ceramiccomposition into the desired shape and then sintering the compositionsin air, followed by reducing the compositions in a reducing atmosphereat elevated temperatures. Ceramics produced from the compositionsdescribed above exhibit a large capacitance among other desirablyelectrical properties and may be conveniently produced in a smallcompact form.

1 Claim, 4 Drawing Figures SEMICONDUCTING CERAMIC COMPOSITIONS DETAILEDDESCRIPTION OF THE INVENTION This invention relates to semiconductingceramic compositions useful for the production of barium titanatesemiconducting capacitors, particularly those capacitors which show verydesirable properties when employed as a by-pass capacitor or in variousdiscriminating circuits. In accordance with this invention, the saidceramic compositions consist predominantly of the non-stoichiometricsolid solution, prepared from a mixture composed mainly of bariumtitanate (BaTiO and, in minor parts, .of suitable amounts of bismuthoxide (Bi o titanate oxide (TiO and manganese (Mn) ions, and which finalceramic product is obtained by firing and reducing the said mixture in areducing atmosphere.

Although semiconducting ceramic capacitors are a relative new field ofinvention, it has now been recognized that they are far superior toconventional insulating ceramic capacitors in that they exhibit a largecapacitance, are small in size and relatively compact, and exhibit otherexcellent characteristics. semiconducting ceramic bodies for capacitorsare classified into the valence control type and the oxidation reductiontype, according to their composition and the method of production.

The semiconducting ceramic bodies of the valence control type arecomposed predominantly of barium titanate, to which are added minoramounts of other elements which have an ionic radius similar to those ofthe constituents of barium titanate and which have a different valency.Since the characteristics of these valence control semiconductors arestrongly affected by the purity of the raw materials, the maintenance ofthe said purity during the manufacturing processes and the necessity ofaccurately weighing the raw materials in combining them in suitableproportions are critical and it is difficult, if not impossible, toprepare such ceramics on an industrial scale. In fact, it is difficultto prepare such ceramics in the laboratory. In addition, the valencecontrol type compositions have other defects in that their specificresistivity cannot be lowered below Qvcm with ease, and also theirelectric properties are limited to the nature of the components whichmake up the ceramic composition.

On the whole, capacitors of the oxidation reduction type are free fromsuch defects as mentioned above but there are some difficulties in thepractical applications of these capacitors. The insulation resistivityof the capacitors of the oxidation reduction type shows a sharp fallwhen the applied voltage is increased and therefore the working voltageof these materials in usual practical application, according to priorart teachings, is near 10 volts with an upper limit of about 12 volts.Another defect of the capacitors of this type is that their barriercapacitive layers are unstable and and undesirable changes in theelectric properties of these materials occurs when lead wires aredirectly soldered to silver electrodes. To prevent this, lead wires areusually attached to the silver electrodes with conductive adhesives. Inpractical uses, when the said capacitors are connected in a circuit,their lead wires are heated to an elevated temperature during thesoldering processes, which sometimes leads to damage of the conductiveadhesives and also to failure in their connections. Particularly, inminiaturized electronic circuits in which lead wires are short, verycareful treatment is needed. Such are the reasons why the semiconductingceramic capacitors of the oxidation reduction type have not been putinto practical use although their utility is well recognized in theory.The need for ceramic materials free from the foregoing defects is wellknown.

It is an object of the present invention to provide ceramic compositionsuseful for the production of semiconducting barium titanate ceramiccapacitors free from the aforementioned defects, that is, semiconductingbarium titanate ceramic capacitors which have a much larger capacitancethan that of the conventional capacitors of the same dimension and whichhave a flat temperature dependence of capacitance over an extendedtemperature range.

A further object is to produce capacitors which show sufficiently highresistivity at an applied voltage higher than that in the case ofconventional semiconducting capacitors and thus are capable of beingutilized in higher voltage regions.

Another object of the present invention is to produce capacitors whichpossess a stabilized barrierv capacitive layer and are free from thefall of their electrical properties.

Another object is to produce capacitors which are free from defects intheir connections at the electrodes, which defects are caused bysoldering lead wires directly, thus allowing a remarkable rise ofefficiency in the manufacturing of the capacitors and in the process ofassembling them in electronic circuits.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a part of the triangulardiagram, which shows the range of compositions of this invention;

FIG. 2 illustrates graphically the temperature dependence of thecapacitance of the capacitors of the compositions of this invention;

FIG. 3 depicts graphically the relationship between the applied voltageand the insulation resistance of the said capacitors, and

FIG. 4 shows graphically the dependence of the insulation resistance andthe capacitance of the said capacitors on the amount of added Mn ions.

The ceramic compositions of this invention will now be illustrated inconnection with the drawings. In FIG. 1, the fundamental constituentBaTiO is located at the point (A); the point (B) corresponds to thecomposition in which BaTiO Bi O is 99 l in molar ratio; the point (C)corresponds tothe composition in which BaTiO Bi O Ti0 is 90 4 6 in molarratio; the point (D) corresponds to the composition in which BaTiO Bi oTiO is 82 4 14 in molar ratio; the point (E) corresponds to thecomposition in which BaTiO B1 0, TiO is 82 2 16 in molar ratio; thepoint (F) corresponds to the composition in which BaTiO Bi,. 0 TiO is 60.l 39 in molar ratio; the point (G) corresponds to the composition inwhich BaTiO "H0, is 60 40 in molar ratio. The region of the compositionsof this invention is the area surrounded by A-B-C-D-E-F-G and Mn ions offrom 0.01 to 4 mol percent with respect to BaTiO may be added to thecompositions in the said region. The raw materials may be oxides orcompounds which give oxides by heating such as carbonates, nitrates,etc. They are weighted and mixed and then sintered in an oxidizingatmosphere. The ceramic bodies thus obtained are reduced and madesemiconductive by firing in a reducing atmosphere. The Mn ions may beadded in any form such as the sulfates, carbonates or nitrates of Mn.The semiconducting ceramic bodies thus obtained are then equipped withsilver electrodes and fired in an oxidizing atmosphere. This final heattreatment simultaneously serves for the plating of the silverelectrodes, the surface diffusion of electrode materials and for thepartial reoxidation of the surface regions of ceramic bodies. All of thesteps described above are the manufacturing methods utilized to producethe semiconducting capacitors of the present invention and areindispensable to provide ceramic bodies with the features and propertiespreviously described.

FIG. 1 is a part of triangular diagram which specifies the ceramiccompositions of this invention. The end members of the diagram are BaTiOBi O and TiO This figure gives the compositions solely with respect tothe said oxide components, and the Mn and oxygen components are omitted.At the same time, the areas of low BaTiO content and high Bi O contentare also omitted. The amounts of the components are given in molpercent. The area surrounded by A-B-C-D-E-F-G is the oxide compositionrange of this invention and the points I, II and III in the said areacorrespond with the Examples I, II and III respectively. This inventionwill be further illustrated by the following examples.

EXAMPLE I.

Oxides were employed as the starting materials. They were combined andmixed so as to give the composition BaTiO 90.74 mol Bi O; 1.85 mol TiO7.41 mol Parts of the oxides may be replaced by carbonates or othersalts. Mn ions were further added to the said mixture in the form of anaqueous solution of manganese sulfate in such amount so that the Mn ioncontent is about 0.18 mol with respect to the amount of BaTiO Distilledwater was then added to the mixture and sufl'iciently mixed in a ballmill containing a polyethylene lining. The obtained mixture was pressedand shaped into disks of a diameter of 13.8 mm and having a thickness of0.5 mm under a pressure of 2,000 Kglcm The shaped bodies were thensintered at l,250 C in air for 2 hours and afterward fired at l,000 C ina hydrogen gas flow for 2 hours. In the latter treatment, the ceramicbodies lose some amount of oxygen and become semiconductive. Then theopposite faces of the obtained semiconductive ceramic bodies Capacitance43000 PF/cm tan 6 5.2% The coefficient of the temperature dependence ofcapacitance between Insulation resistance 80 Mfllcm (all the quantitieswere measured under the same conditions as in Example 1).

FIG. II graphically illustrates the temperature dependence of thecapacitors in the above-mentioned Examples 1, II and 1 III. The curvesIa, Ib and lo correspond with Example I, II and 15 ties of thesemiconducting ceramic capacitor materials, ex-

pressed in Examples I, II and III, and the above descriptions, areattained solely in the compositions described within the areaA-B-C-D-E-F-G in FIG. 1. The ceramic compositions of this invention aretherefore limited within the said area. Table were painted with silverelectrode paste. These ceramic bodies 20 1 gives the compositions andthe properties of the semiconwere heated in air up to about 800 C andthus the painted ductor ceramic capacitors prepared from variouscomposisilver electrode material was affixed on the faces of theceramtions outside the limited area utilizing the same method of icbodies. Finally, lead wires were directly soldered on the surpreparationdescribed in Example I.

TABLE 1 li1ili03 BlzO; 'PIOQ Mn ion Insulation Ninnlmr of (mol. (mol.(lnol. (mol. (lupuuittnme, 'Iun, resistance spvvnnml pot-cunt) percent)purcont) pnrcont) PIP/0111.2 pore-(int (Mil/0111.

1 01.23 2. s0 2. x0 1. 0 1.1 10 z 3, 000

faces of the silver electrodes by immersing them in fused solder. Thecharacteristic values of the semiconducting ceramic capacitors thusobtained are as follows:

61000 PF/cm Capacitance The coefficient of temperature dependence ofelectric capacitance between and +85C +25-8% (all of the abovequantities were measured at 25 C at l KH,)

Insulation resistance (measured at 25C under the applied voltage of 20volts).

EXAMPLE II.

characteristic values of the semiconducting ceramic capaci- 55 tors thusobtained are as follows:

Capacitance 52000 PF/cm tan 5 4.6% The coefficient of the temperaturedependence of capacitance between -50 and +85C +25--l 1% Insulationresistance 4000 MIT/cm.

(all of the above quantities were measured under the same conditions asemployed in Example I).

EXAMPLE III.

In this example, oxides were employed as the starting materials andcombined and mixed so as to form the composition: BaTiO 69.72 mol Bi O0.40 mol and TiO 29.88 mol percent. Manganese sulfate was then added tothe mixture in such amounts so that the Mn ion content is about 0.697mol percent with respect to the mol of BaTiO The method of preparationand other conditions were the same as in Example I. The characteristicvalues of the semiconducting ceramic capacitors are as follows:

35 the increase of the amount of excess Bi O that dense ceramic bodiescannot be obtained with ease unless a binder is employed. Aninvestigation with the X-ray powder diffraction method has revealed thatthe ceramic bodies obtained when the Bi,o, content is in excess containsfree B00, in addition to 4 a in addition to a solid solution of bariumtitanate. Besides this defect, the ceramic bodies withexcess Bi O have atendency to break off during the treatment to make them semiconductive,thus making it difficult to produce semiconductive ceramics having thedesired shape. Particularly, the ceramic 45 bodies of the composition inwhich the B1 0, is so excessive that it remains in an independent phase,break off so easily that it is impossible to make them semiconductive.Even in the composition region, where the amount of the Bi O is not sohigh, the ceramic bodies break if the amount of Mn ion is increased. Inthe case of the specimen 1 of the Table l, which retains the shapewithout breaking off during the heat treatment, the capacitance is quitelow, although the insulation resistance still shows a large value.

Thus, this specimen loses a feature of semiconducting ceramic capacitorsand is not suited for practical applications. It has been found out thatthis decrease of capacitance is caused by the fact that the reoxidationduring the process of plating electrodes progresses too rapidly in thiscomposition range. This tending to decrease in capacitance is exhibitedduring the manufacturing process in the composition region where the Bio content is greater than that described by the line C-D mentionedabove. In some cases, the shape of the ceramic bodies is retained, buteven at that time the capacitance of the ceramic bodies becomes quitelow, owing to strong reoxidation. Thus, the compositions in the saidregion, i.e. above the C-D line, are also not suited for practicalapplications, as illustrated by the specimen 2 in the Table 1. In thecompositions about the point D, the existence of another compound Ba BiTi O besides BaTiO has been recognized in the case where Bi O is used inexcess of thatcalled for in the present invention. The specimen 3 in theTable l is an example of ceramic compositions in the region where theBaTiO content islower than on the line D-E. Such compositions areunsuited for practical applications because of a fall of capacitance anda strong increase in the tan8 value. The region where the BaTiO contentis lower than on the line DE is characterized by the existence of thecompound Ba Bi Ti O The coexistence of this compound is accompanied by asharp increase in tano, as illustrated by specimen 4 of Table l, whichis located in the composition region where the Bi O content is in excwsof the proportions described by the line E-F. As for the region wherethe Bi O content is lower than that described by the line E-F, anexamination by the X-ray powder diffraction method has revealed thatsmall amount of another compound exists. However, the existence of Ba BiTi O has not been confirmed by this method. In fact, the ceramic body ofa composition in this range possesses excellent properties asillustrated in Example III. The properties of the semiconductingcapacitors of this invention are not influenced by the existence ofsmall amounts of other compounds and this is one of the surprisingfeatures of the capacitors. In the region wherethe BaTiO content islower than on the line F-G, however, the amount of the otheraforementioned coexisting compound becomes so large that the value oftan8 increases, as seen in the case of specimen 5 in Table 1. Thus,ceramic bodies falling within these composition regions are unsuited forpractical applications.

From the detailed descriptions given above, it can be concluded thatceramic bodies having excellent characteristics in regard tocapacitance, tano and insulation resistance can be prepared fromcompositions containing constituent oxides lying in the area surroundedby A-B-C-D-E-F-G in FIG. 1. Further, it has been discovered that theseproperties are not lost when a part of the said composition is replacedwith a small amount of magnesium titanate, calcium titanate or otherdouble oxides of the A30 type.

The effect of adding Mn ions to compositions within the aforementionedregion will now be described. FIG. 3 graphically shows the relationshipbetween the applied voltage and the insulation resistance of thecapacitors described in Examples I, II and III. The curves Ila, llb andHe correspond with the Examples I, II and Ill respectively. For thepurpose of com parison, a corresponding curve for a semiconductingcapacitor prepared by the same method and from the same ceramiccomposition as described in the Example I except without the addition ofMn ions is also shown in the curve IId. These curves show that theaddition of Mn ions produce capacitors having better insulationresistance than compositions wherein Mn ions are left out.

FIG. 4 shows the relation between the insulation resistance and theamount of added Mn ions for semiconducting capacitors prepared by thesame means and from the same c omposition as in Example I. The verticalaxis on the left side shows the value of the insulation resistance andthe one on the right illustrates the value of the capacitance. Thecurves Illa and Illb shows the insulation resistance and the capacitancerespectively. As seen from this figure, the addition of Mn ions, even ina small amount, brings about a rapid increase in the insulationresistance. It is thought that this increase is caused by the loweringof the conduction electron density in the barrier layer, owing to thevalency compensation effect of Mn ions in the barrier layer.

In the case where the amounts of added Mn ions is less than 0.01 molpercent, the insulation resistance falls below 10 Mil/cm and theimprovement of the insulation resistance by the Mn ion addition isineffective. On the other hand, when the amount of added Mn ionsnexceeds 4 mol percent, the capacitance becomes quite small, althoughthe insulation resistance is high. Thus, the possibility of attaining acapacitor having a large capacitance and small size is lost. The effectof the addition of Mn ions described above is similar at any part of theaforementioned composition described by the region A- B-C-D-E-F-G. Theamount of Mn ions added to compositions falling within theaforementioned region is limited from 0.01 to 4 mol percent.

As understood from the detailed description given above, ceramiccapacitors produced according to the present invention are superior tothe capacitors in prior art in several respects. That is, they have alarger capacitance and small temperature dependence of capacitance. Inaddition, the capacitors show high insulation resistance when voltage isapplied, thus being able to be widely employed in the circuits operatedat high working voltages. Besides the excellent electric propertiesmentioned above, the said capacitors may be manufactured efficiently andalso electronic circuits utilizing these capacitors may be easilyconstructed since lead wires can be directly soldered on theirelectrodes. Thus, miniaturized electronic circuits may be efficientlyproduced by utilizing capacitors of the present invention.

What is claimed is:

Compositions useful for producing semiconducting ceramic capacitorsconsisting essentially of BaTiO with small amounts of Bi O and also TiOadded thereto, which composition ranges fall within the polygonal areadescribed by the line A-B-C-D-E-F-G in FIG. 1, and in which Mn ions areadded to said composition within the range of 0.01 to 4 mol percent.

